Of thermoplastic halocarbon polymers, fluorocarbon polymers generally have superior resistance to solvents and absorb no water at all. In addition, fluorocarbon polymers are quite excellent in weather resistance, heat durability, abrasion resistance, and nonstickiness of all plastics.
Polytetrafluoroethylene (PTFE) is a typical fluorocarbon polymer. Because of its high resistance to heat and solvents, PTFE is used as an anticorrosive coating on the inner surfaces of pipes and as packings in chemical apparatuses. Since PTFE has a low dielectric constant, a low dielectric loss, and a high electrical resistivity, it is used as an insulator on electric cables. PTFE is also used for oilless retainer and guide shoe because it is excellent in surface lubricity. However, since PTFE has a high melting point and cannot be easily molded by melt-forming techniques, disadvantageously PTFE must usually be formed by sintering procedures. With a view to improving the moldability of PTFE, copolymers of tetrafluoroethylene with other unsaturated monomers or fluorocarbon monomers other than tetrafluoroethylene have been proposed. Furthermore, non-tetrafluoroethylene type fluorocarbon polymers which are thermoplastic and yet exhibit properties similar to those of PTFE have been developed.
Despite their superior properties, PTFE and the improved fluorocarbon polymers are expensive and for general purposes it is considered useful that they are laminated with inexpensive substrates.
The problem, however, is that fluorocarbon polymers are non-adhesive and involve considerable difficulty in terms of being bonded to substrates.
A variety of techniques have been studied in order to provide fluorocarbon polymers with improved adhesion to substrates. One approach is to modify the surfaces of fluorocarbon polymers either by the wet process wherein the surfaces of molded fluorocarbon polymers are treated, for example, with a solution of an alkali metal or by dry processes such as corona discharge, plasma discharge and sputter etching. According to other approaches, the surface of a fluorocarbon polymer is eroded by a special solvent that can dissolve the polymer and is bonded to substrates; alternatively, a fluorocarbon polymer is physically bonded to substrates by sandwiching a glass mat.
Adhesive polymers that can be used as substrates to be bonded to fluorocarbon polymers have also been proposed. There are known, for example, certain ethyleneethyl acrylate copolymers, ethylene-vinyl acetate copolymers, and modified products thereof (see Japanese Patent Application Laid-Open Nos. 86748/81 and 12645/82); epoxy group-containing polyolefins (see Japanese Patent Application Laid-Open Nos. 8155/82 and 212055/82); and polymer compositions comprising methyl methacrylate polymers and copolymers grafted with vinylidene fluoride (see Japanese Patent Application Laid-Open No. 12646/82).
Chlorocarbon polymers as thermoplastic halocarbon polymers are generally flame-retardative, have superior resistance to water, acids and alkalis, are stable in many organic solvents, and exhibit superior barrier properties against various gases and water vapor. Vinyl chloride polymer (PVC) and vinylidene chloride polymer (PVDC) are typical chlorocarbon polymers.
Vinylidene chloride homopolymer has a softening point which is very close to the thermal decomposition temperature and the plasticizers and stabilizers that can be used with this polymer are limited. Therefore, in order to provide PVDC having improved moldability, vinylidene chloride is copolymerized with such monomers as vinyl chloride, acrylonitrile and acrylates. Such copolymers (PVDC) are used in textile products such as fishing nets, insect-screen nets, and tents, as well as in packing films, and coating latices. PVDC has superior gas barrier properties but it is expensive and does not exhibit satisfactory degrees of heat resistance and stiffness when used alone. Therefore, in the food packing industry, PVDC is used in the form of dispersion coatings or, alternatively, it is laminated with substrates, for example, of styrene type resins by coextrusion molding, and they are used for films and containers. In the case of containers, a dispersion of PVDC is difficult to apply and requires a drying process. In order to avoid this complexity of operations, it is desirable to fabricate PVDC containers by the simpler coextrusion molding rather than the dispersion coatings.
PVDC is not highly adhesive and attempts have been made to improve the adhesive properties of its dispersion to the substrates by introducing functional groups through copolymerization with other monomers.
Adhesive polymers that are suitable for use as substrate which adhere to PVDC have been proposed and they include: ethylene-vinyl acetate copolymers (see Japanese Patent Application Laid-Open Nos. 66770/74 and 26876/78; and Japanese Utility Model Application Laid-Open No. 83339/78); thermoplastic polyurethanes (see Japanese Patent Application Laid-Open Nos. 106584/79 and 217151/85); and certain polyesters (see Japanese Patent Application Laid-Open No. 152085/79).
However, these prior techniques are not completely satisfactory in terms of moldability, workability and adhesive strength and the types of substrates to which they can be bonded are limited. For example, as for fluorocarbon polymers, Japanese Patent Application Laid-Open Nos. 86748/81 and 12645/82 disclose laminates of thermoplastic fluorocarbon polymers and ethylene-vinyl acetate copolymers; if, with a view to providing improved adhesion, a large amount of vinyl acetate is introduced in the form of a copolymer with ethylene, the resulting copolymer has a lowered mechanical strength and yet its adhesive strength to the fluorocarbon polymer is not significantly improved. As for chlorocarbon polymers, most cases of coextrusion molding are laminates of PVDC and ethylene-vinyl acetate copolymers as proposed in Japanese Patent Application Laid-Open Nos. 66770/74 and 26876/78 and Japanese Utility Model Application Laid-Open No. 83339/78, but ethylene-vinyl acetate copolymer adheres poorly to styrene polymers and has not been suitable, for example, for use as industrial parts, and food packaging materials.